1. Field of the Invention
The present invention relates to a glass electric lamp bulb the inner surface of which has been coated to provide a low cost and environmentally safe diffusing coating having a visual appearance similar to that produced by conventional hydrofluoric acid etching.
2. Description of the Prior Art
For most lighting applications, it is desirable to provide some form of light diffusion in the outer bulb to reduce glare and produce a softer, more even illumination. Conventional incandescent lamps have a thin-walled outer bulb which is hermetically sealed to the bulb base in order to contain the inert gas fill which is the necessary environment for proper operation of the incandescent filament. One known method of providing the desired light diffusion in such a bulb has been by hydrofluoric acid etching or "frosting" of the inner surface of the bulb. This process is relatively costly, and causes special concern regarding the proper disposal of fluoride-containing waste.
Manufacturers of incandescent lamps have provided light diffusion by a "smoke" coating of the inner bulb surface with fumed silica. Fumed silica may comprise silicon dioxide formed, for example, by flame hydrolysis of silicon tetrachloride. The discrete particles formed thereby are extremely fine, having a diameter on the order of the wavelength of visible light. These "smoke" coatings tend to be white in appearance, and are somewhat less optically efficient than acid frosted bulbs, but they do alleviate the problem of hazardous waste disposal created by the acid process.
Manufacturers have made other attempts to overcome the problems incurred in providing light diffusion in bulbs. In U.S. Pat. No. 2,661,438 to Shand, which issued on Dec. 1, 1953, a mixture of alkaline-reacting silica aquasol and silica aerogel or silica xerogel is used for coating of incandescent lamp envelopes.
In U.S. Pat. No. 3,175,117 to Kardos, which issued on Mar. 23, 1965, floccular titanium dioxide, silicon dioxide or red iron oxide is mixed with granules of titanium dioxide in a solvent and applied to the inner bulb surface.
Silica is deposited on the inner surface of glass bulbs by the oxidation of silane with oxygen in U.S. Pat. Nos. 4,374,157 and 4,438,152 to Barbier et al, which issued on Feb. 15, 1983 and Mar. 29, 1984, respectively.
U.S. Pat. Nos. 2,963,611 and 2,922,065 to Meister et al., which issued on Dec. 6, 1960 and Jan. 19, 1960, respectively, teach that the addition of a limited amount of a finely-divided white material having a true density relatively high with respect to the density of silica will improve the adherence of the silica to the inner surface of the bulb. Such materials may include titania, barium titanate or zirconia.
The basic silica smoke process as described above is taught in U.S. Pat. No. 2,988,458 to Meister et al., which issued on June 13, 1961.
A similar smoke process is disclosed in U.S. Pat. No. 4,099,080 to Dawson et al., which issued on July 4, 1978. Improved adherence and freedom from agglomerations of the silica are provided by utilizing a mixture of hydrophilic silica and hydrophobic silica.
More effective light scattering by the silica coating is provided by the addition of a thin layer of spherical alumina particles in U.S. Pat. Nos. 3,842,306 and 3,868,266 to Henderson et al, which issued on Oct. 15, 1974 and Feb. 25, 1975, respectively.
In U.S. Pat. No. 3,909,649 to Arsena, which issued on Sept. 30, 1975, control of the size of the silica particles and the use of a polyacrylic acid binder in an ammoniacal water solution results in yet another improvement in the silica coating process.
The electrostatic coating processes taught by Collins and James in U.S. Pat. Nos. 4,081,709, 4,441,046 and 4,441,047, which issued on Mar. 28, 1978 and Apr. 3, 1984, respectively, illustrate still further attempts to improve upon the basic silica smoke coating process.
Recent developments in the lighting field have led to increased use of tungsten halogen lamps. Such lamps generally utilize a relatively heavy or thick outer bulb or envelope which is designed to contain any glass fragments in the event of rupture of the inner tungsten halogen capsule, which may operate with a hot fill pressure of ten atmospheres or more.
These heavy-walled, molded bulbs tend to have somewhat nonuniform glass thickness as well as surface marks such as mold closure lines, and the application of a white interior diffusing coating such as the silica coatings described above produces an objectionable appearance which accentuates the bulb thickness and reveals all nonuniformities and marks. Additionally, since the outer bulbs of tungsten halogen lamps are not hermetically sealed, ambient atmospheric moisture tends to diminish the adherence of silica coatings to the inner surface of the bulb, and mechanical shock and vibration can cause detachment of the silica particles.
In contrast, when the acid etch process is used on the inner surface of the outer bulb of a tungsten halogen lamp, a rough surface texture which is optically integral with the glass is created, which texture tends to conceal both the thickness and nonuniformity of the bulb wall, as well as exterior surface marks such as mold lines. However, as noted such a process is environmentally hazardous.
The prior art does not provide a light diffusing coating on the inner surface of a bulb for a tungsten halogen lamp or the like which is operationally effective, aesthetically pleasing and can be applied in a manner which is environmentally safe. It is an object of the present invention to provide a glass electric lamp bulb such as, for example, a bulb for use in a tungsten halogen lamp, having a light-diffusing coating which overcomes these problems.